To determine the presence of hydrocarbons in different formations traversed by a borehole, it is conventional to make several different logging measurements at different depths along the borehole. The measurements are then processed to determine values of output parameters indicative of specific borehole characteristics which are more meaningful than the actual measurements but which are not directly measurable themselves. Typical examples of output parameters are porosity .phi., water saturation S.sub.w and clay content V.sub.cl. Examples of some initial, well known, logging measurements include: density .rho..sub.b, "neutron" porosity .phi..sub.N, acoustical transit time .DELTA.t, natural radioactivity GR, spontaneous potential SP, and various resistivity measurements which laterally penetrate into the formation to different degrees. Each of these measurements are made at a multiplicity of successive levels of the borehole.
Known interpretation methods include a certain number of preliminary processing steps designed essentially to provide corrections and to determine a plurality of borehole zones in which indications of specific borehole characteristics have substantially constant values. Indentification of borehole zones is then followed by the interpretation of each zone, level by level.
A level-by-level interpretation method is described in co-pending application Ser. No. 112,005 filed Feb. 1, 1971, now abandoned, and continued as Ser. No. 060,835, filed July 25, 1979. A corresponding patent has issued as French Pat. No. 2.102.380 (Poupon-Gaymard). Briefly stated, that method consists of evaluating the clay content at each level, correcting the measurements for the clay effect and, assuming values for porosity .phi. and for the residual hydrocarbon saturation S.sub.hr, determining a first hydrocarbon correction which makes it possible to find a first value of .phi.. This first value of .phi. is used for calculating new hydrocarbon corrections which give an improved value of the porosity .phi.. The water saturation S.sub.w is then calculated from the porosity values .phi. and resistivity values R.sub.t of the formations. Several iterations can be applied to obtain good values of .phi. and .rho..sub.ma, the matrix density.
In prior-art methods, certain measurements are chosen and combined to find a first indication of a borehole characteristic, hereafter more conveniently referred to as an output parameter. This output parameter is then combined with new measurements to obtain a subsequent, different output parameter. As is evident, these two output parameters are obtained successively. Use is sometimes made of iterative methods to reintroduce into the calculations an output parameter with better accuracy.
These methods have yielded good results but nevertheless have their drawbacks. A first drawback is that it is difficult to modify the above described technique to take advantage of new or different measurements. For example, if geological or other data derived from cores are known, they cannot be introduced into the interpretative process unless provision has been made at the outset for a certain number of options allowing this to be accomplished. A second drawback is that the prior art methods may be unable to consider a number of measurements of varying quality, some of which could be used to advantage.
It is therefore the object of the invention to provide a method for evaluating formations traversed by a borehole, said method taking into consideration most, if not all, of the information available, including "a-priori" information.
It is another object of the invention to provide a method for evaluating formations which takes into account the uncertainity with which each measurement is obtained. Thus, a measurement having a low precision will participate in the interpretation but with little influence. In this way, no information is neglected and the various information is given only the influence it deserves.
It is a further object of the invention to provide a method which produces new output parameters and new presentations for both the output parameters and the initial measurements.